Responses Of Physiological And Biochemical Traits To The Change Of CO₂Concentrations And Temperature In Marine Macroalgae

CO₂concentration and temperature are critical environmental factors affectingphysiological and biochemical traits of marine macroalgae. At present, there are many studieson the responses of photosythetic and biochemical traits of marine macroalgae to elevatedCO₂, and the effects of temperature on marine macroalgae are paid increasing attention.However, little is known about the impact of CO₂combined with temperature on macroalgae.This study was trying to explore the combined effects of CO₂and temperature on the growthand thermal characteristics in Ulva lactuca （Ulva, Chlorophyta）, Porphyra haitanensis（Bangiales, Rhodophyta） and Gracilaria lemaneiformis （Gigartinales, Rhodophyta）. Theprimary results were as follows:1) Elevated CO₂enhanced the growth of U. lactuca. The effects of CO₂on solubleprotein （SP）, soluble carbohydrate （SC）, chlorophyll a （Chl a） and carotenoid （Car） weretemperature-dependent. After exposed to10℃for6h, photoinhibition occurred in U.lactuca. After exposed to35℃for6h,25℃and normal air-grown algae displayedhighest optimal photochemical quantum yield （Fv/Fm）, photosynthetic light use efficiency （α）and non-photochemical quenching （NPQ）; while25℃and high CO₂-grown algae exhibitedhighest maximum relative electron transport rate （rETRmax） and light saturation point （Ek）.After exposed to40℃for6h, photosynthetic apparatus of15℃-grown algae wereseriously destroyed; For25℃growth condition, high CO₂-grown algae displayed lowerdecline degree in Fv/Fm, a, NPQ and photochemical quenching （qP） compared to normalair-grown. Our results suggested that elevated CO₂improved heat endurance in U. lactuca.2)15℃-grown P. haitanensis had higher relative growth rate, Chl a and Car than25℃-grown algae, with higher enhancement extent under growth condition of high CO₂relative to normal CO₂. Regardless of CO₂concentrations,15℃-grown algae showed higherlevels of SP and SC than25℃-grown algae. After treated with10℃-stress for3h,15℃-grown algae exhibited slower decline in Fv/Fm, α and NPQ than25℃-grown. After treatedwith35℃-stress for3h, the rETRmax, Fv/Fm and α were decreased, with higher decreaseextent in15℃-grown algae than25℃-grown algae. When treated with40℃-stress for20min, the rETRmax, Fv/Fm, α, NPQ and qP were decreased drastically. The results suggestedthat elevated CO₂improved heat endurance in P. haitanensis.3) High CO₂, high temperature and greenhouse effect （high CO₂combined high temperature） enhanced the growth of G. lemaneiformis, with the highest enhancement forgreenhouse effect. Greenhouse effect increased photosynthesis rate （Pn） and dark resperationrate （Rd） but decreased the contents of SP and SC in algae. Both contents of Chl a and Carwere increased with high temperature; while the contents were unaltered with high CO₂.Either increased CO₂or temperature had no significant effects on the contents of PE and PC.The changes of Fv/Fm、α、Pn and Rd under high-temperature stresses showed the sametendency, i.e: their values all increased slightly under32℃, but decreased under36℃, anddeclined fiercely under40℃. The high temperature tolerance limit of20℃-grown algaewas between32℃and36℃, while that of24℃-grown algae was between36℃and40℃. Our results suggested that elevated CO₂or temperature showed no positively effect onPn and Rd, while greenhouse effect improved high temperature resistance.In conclusion, our results showed elevated CO₂combined with temperature affectphysiological and biochemical traits in marine maroalgae, with the effects beingspecies-dependent. High CO₂improved the heat endurance in both U. lactuca and P.haitanensis. Moreover, high CO₂combined high temperature improved high temperatureresistance of G. lemaneiformis. These might be essential for the physiological function inmarine maroalgae in the context of climate change.